The invention relates to multi-component dynamometers for measuring forces and/or torques, especially during metal-cutting operations, consisting of at least one piezoelectric transducer element between a pair of force introduction plates.
Dynamometers for measuring forces and torques are known (e.g. DT-PS No. 9 57 980). They are relatively complicated devices, provided with six force transducers each sensitive selectively in one direction, which measure the forces and the torques resolved into forces The measuring elements used do not measure forces directly but strains This has the detrimental consequence that the two machine components, in themselves rigid, between which forces and torques are to be measured, are joined elastically. As a result the measuring system has a relatively low natural frequency, so that rapid changes of the measured variables as occur especially in metal-cutting operations are no longer detectable. Moreover, the force introduction is complicated with these systems, because the force resolution into the six components dictated by the measuring arrangement is effected typically via elastic knife edges. For these reasons, such devices are of complicated design and cannot be assembled into compact components.
Similarly complicated would be fitting force transducers as described in the aforementioned DT-PS with piezoelectric measuring cells instead of the strain gauging systems capable of registering a force acting in one direction only. The resulting devices are too bulky and complicated. Simpler is the fitting of systems for measuring forces and torques when multi-component measuring platforms are used as described in DT-PS No. 19 52 522. These platforms contain four force-measuring cells in individual housings, each responding to force components, generally more than one. Arrays consisting of 6 piezoelectric crystals may be used as measuring cells, one pair sensitive to pressure and two pairs to shear, generally in directions normal to each other. Such measuring cells are described, for example, in "Piezoelektrische Messtechnik", Springer Verlag, 1980, p. 169.
Though dynamometers designed in this way have adequate rigidity for many applications, higher natural frequencies are desirable especially in metal-cutting operations, in order to detect still faster changes in the force components. Moreover, still flatter dynamometers are desirable, in order to minimize their mechanical elasticity and to require as little space as possible for installation. In addition, mounting these dynamometers should be as simple as possible.
The invention solves the task arising from these desiderata by virtue of the following features according to which the multi-component dynamometer contains transducer elements each having at least two piezoelectric elements joined by an insulating carrier arranged between them. The piezoelectric elements are pre-oriented in relation to the coordinate system given by the carrier, whereby one of the piezoelectric elements of each transducer element is pressure-sensitive in a direction normal to the plane of the force introduction plates while the other piezoelectric element is shear-sensitive in the plane of the force introduction plates.
The shear sensitivity directions of the individual transducer elements may be chosen differently within the plane of the force introduction plates.
Unlike the above-mentioned DT-PS No. 19 52 522, the preferably non-housed transducer element of the dynamometer according to the invention is pre-oriented crystallographically, and in the preferred embodiment with one shear-sensitive piezoelectric element, it responds to only one shear component and not to two components. The invention provides one part of the transducer elements of the dynamometer sensitive to shear in one direction, and the other part to shear in the direction normal thereto. The two directions span a plane XY at right angles to the normal direction of the force introduction plates (Z axes). In this direction, the transducer elements are sensitive to pressure.
The invention enables a dynamometer to be made considerably flatter than conventional ones, because each transducer element consists of only two piezoelectric elements and a carrier plate between them. Since the force may be introduced equally through the transducer elements described as through additional substitute elements, while both these and the transducer elements are of hard material, a mechanically very rigid measuring platform results, which can be fitted directly between machine parts and which has a very high natural frequency. It is therefore eminently suited for dynamic measurements during metal-cutting operations, especially where very rapid changes of force and torque components may occur.